1. Field of the Invention
The present invention relates to a semiconductor device having transistors and a driving method thereof. In particular, the invention relates to a semiconductor device having pixels each including a thin film transistor (hereinafter also called a “TFT”) and a driving method thereof.
2. Description of the Related Art
In recent years, a thin display (also called a flat panel display) using elements which use the electrooptic property of liquid crystals or emit light with electroluminescence has been drawing attention and the market of such industries is expected to enter into the expansion phase. So-called active matrix displays where pixels are formed with TFTs over a glass substrate have been gaining importance as a thin display. In particular, a TFT having a channel portion formed of a polycrystalline silicon film can achieve a high-speed operation since it has higher electron field-effect mobility in comparison with a conventional TFT using an amorphous silicon film. Therefore, the pixels can be controlled with a driver circuit which is formed by using TFTs over the same substrate as the pixels. A display where pixels and a functional circuit are formed over the same substrate by using TFTs has various advantages such as reduction of component parts, improvement in yield by the simplified manufacturing process, and improvement in productivity.
An active matrix display where electroluminescence elements (hereinafter also called “EL elements” in this specification) and TFTs are combined (hereinafter also called an “EL display”) can achieve reduction in thickness and weight; therefore, it has been drawing attention as a next-generation display. Such a display is examined to be developed to displays with various sizes, for example from a small size of 1 to 2 inches to a large size of over 40 inches.
Luminance of an EL element has a proportional relationship with the amount of current flowing therein. Therefore, an EL display which uses an EL element as a display medium can express gray scales by using current. As a method for expressing gray scales, a method of controlling the amount of current flowing in an EL element is known, where the EL element and a TFT (hereinafter also called a “driving TFT”) are connected in series between two power supply lines, and a gate-source voltage of the driving TFT operating in the saturation region is changed to control the amount of current flowing in the EL element (for example, see Reference 1: Japanese Patent Laid-Open No. 2003-271095). In addition, there is also a driving method for expressing gray scales by using a constant current and controlling the time when the current flows in an EL element (for example, see Reference 2: Japanese Patent Laid-Open No. 2002-514320).
However, the conventional pixel configuration has a problem in that power consumption is increased if a potential of a wire for outputting a video signal (hereinafter also called a “signal line”) changes every time a video signal is applied to a gate of a driving TFT (driving transistor) from the signal line, since the parasitic capacitance of the signal line stores and releases electric charges.